Late last year Microsoft released free versions of it’s Office mobile apps for iOS (iPhone and iPad) – Word, Excel, Powerpoint, and now Outlook. You can read about the original release here and a more up to date article here.
Originally, these apps only worked with documents stored locally or on Microsoft’s OneDrive cloud solution. Now, this restriction has been lifted and these apps work with documents stored on other cloud services like Dropbox. This means, for example, that you can work on an Excel document on your computer, save it to Dropbox, and then view or edit it on your iPhone or IPad with the Excel app.
You can download all these apps from the Apple App store.